TDM and FDM MCQ || TDM and FDM Questions and Answers

11. Serial transmission is not possible without ________

  1. TDM
  2. FDM
  3. PDM
  4. None of the above

Answer.1. TDM

Explanation

Time-division multiplexing of multiple signals is possible only when the available data rate of the channel exceeds the data rate of the total number of users. While TDM may be applied to either digital or analog signals, in practice it is applied almost always to digital signals.

The data transmission may be serial or parallel. In serial transmission, bits are transmitted serially whereas in parallel transmission, all bits of a byte are transmitted at a time.

Parallel transmission is used for the devices which are close to each other whereas serial transmission is used for transmission over long distances. The communication modes allow for one-way, two-way ordered, or two-way simultaneous transmission.

In serial transmission, the data is sent via a single cable. When a clock pulse is applied to the shift register it transmits the information bit by bit in allocated time slots.

 

12. A TDM link has 24 signals and each channel is sampled 8000 times/sec. Each sample is represented by 8 binary bits and it contains an additional bit for synchronization. The total bit rate made from the TDM link is

  1. 1600 K bits/sec
  2. 1728 K bits/sec
  3. 1826 K bits/sec
  4. 2056 K bits/sec

Answer.2. 1728 K bits/sec

Explanation

Given sampling rate of channel each = 8000 Hz = fch

Since 24 such signals are Multiplexed, we get a total pulse per second (frequency) of the multiplied signal as:

fs = nfch

Putting on the respective values:

fs = 24 × 8000

fs = 192,000 = 192 KHz

Now each sample is represented by 8 bits and contains an additional bit for synchronization (n).

The total no. of bits per sample will be:

n = 8 + 1 = 9 bits

∴ The total bit rate of TDM link is given as:

Rb = nfs

Rb = 9 × 192 Kbps

Rb = 1728 Kilo-bits/sec

 

13. 24 voice channels are sampled uniformly at a rate of 8 kHz and then time division multiplexed. The sampling process uses flat-top samples with 1 μs duration. The multiplexing operation includes the provision of synchronization by adding an extra pulse of 1 μs duration. The spacing between successive pulses of the multiplexed signal is:

  1. 4 μs
  2. 6 μs
  3. 7.2 μs
  4. 8.4 μs

Answer.1. 4 μs

Explanation

Given that,

The total number of channels is the sum of the voice channels and synchronized samples

Total number of samples n = 25

Sampling frequency fs = 8 kHz

Sampling time Ts = 1 / fs

⇒ Ts = 1 / 8000

∴ Ts = 125 μs.

Time duration utilized by the samples is

T = Ts / n

⇒ T = 125 / 25

T = 5 μs

The pulse duration is 1μs

The spacing between successive pulses of the multiplexed signal is = 5 – 1 = 4 μs

 

14. The main advantage of time-division multiplexing (TDM) over frequency division multiplexing (FDM) is that it:

  1. Requires less bandwidth
  2. Requires less power
  3. Requires simple circuitry
  4. Provide better signal-to-noise ratio

Answer.4. Provide better signal-to-noise ratio

Explanation

TDM (Time Division Multiplexing):

It is a multiplexing technique that allows the transmission of multiple signals over a common channel but in different time slots.

Each signal will get transmitted very quickly over the channel but at a time only one signal will be transmitted.

FDM (Frequency Division Multiplexing):

It is also a multiplexing technique like TDM, in which multiple transmitted signals use a common channel but the total available bandwidth is utilized among the various signals.

This implies that over a complete channel a particular frequency slot is allotted to only one signal.

In FDM, a different frequency band is used to modulate different data signals. This means that different carrier frequency modulates the various signals that are to be transmitted over the channel.

Key differences between FDM and TDM:

FDM TDM
It is used in Analog signal transmission It is used in Digital signal transmission
In FDM the signals are allotted different frequency slots, this leads to the possibility of crosstalk while signal transmission. In TDM multiple signals are transmitted in different time domains, thus the chances of crosstalk between the signals are negligible
The Signal-Noise ratio is higher Provide better Signal-Noise ratio
Synchronization is not needed Synchronization is necessary
Circuit orientation is quite complex Circuit orientation is comparatively simple.
Costlier Comparatively less
FDM offers low efficiency because the technique is more prone to cause interference while transmission. TDM is highly efficient because the chances of interference are less in its case.
No propagation delay Propagation delays are present

 

15. Random-access Protocol is

  1. Distributed
  2. Fault-tolerant
  3. Distributed & Fault-tolerant
  4. None of the mentioned

Answer.3. Distributed & Fault-tolerant

Explanation

Random Access protocols deal with the access to the transmission medium in communication networks, as part of the medium access control layer. Random access is simple, distributed, and very fault-tolerant.

 

16. What device is used to demodulate a time division multiplexed analog wave?

  1. High pass filter
  2. Low pass filter
  3. Bandstop filter
  4. Attenuator

Answer.2. Low pass filter

Explanation

  • In TDM, each signal occupies the entire bandwidth of the channel. However, each signal is transmitted for only a short period of time. A rotating switch called a commutator connects the output of each channel modulator to the input of the communication channel in turn.
  • The commutator is realized with electronic switches since it has to rotate at high speeds. The commutator remains at each contact for a fixed interval of time, which is the time slot allotted for each channel.
  • At the receiver, another switch is used, rotating in synchronization with the commutator in the sending end. This switch connects the pulses received to the appropriate demodulator.
  • A low-pass filter in conjunction with a diode acts as a simple demodulator for an amplitude-modulated wave.
  • The original analog signal is recovered by passing it through a low-pass filter. In TDM using PAM, a circuit called a multiplexer (MUX or MPX) samples multiple analog signal sources; the resulting pulses are interleaved and then transmitted over a single channel.

 

17. Four voice signals, each limited to 4 kHz and sampled at Nyquist rate, are converted into binary PCM signals using 256 quantization levels. The bit transmission rate for the time-division multiplexing signal will be

  1. 8 kbps
  2. 64 kbps
  3. 256 kbps
  4. 5126 kbps

Answer.3. 256 kbps

Explanation

Transmission rate (R = nmfs

n → number of bits in the signal

m → number of signals

fs → sampling frequency

Calculation:

Given:

No. of levels of PCM = 256

∴ no. of bits = log256

= 8 bits

m = 4

fs = (2fm) = 8 kHz

(R = (8 × 4 × 8)

(R = 256 kbps

 

18. What multiplexing method do the CODECs use to transmit two PCM signals in opposite directions over the same transmission line?

  1. TDM
  2. FDM
  3. Amplitude-separation Multiplexing
  4. Code Division Multiplexing

Answer.1. TDM

Explanation

CODEC is a device that can simultaneously perform the analog-to-digital conversion (encoding) and digital-to-analog conversion (decoding).

Time Division Multiplexing (TDM)

  1. A process that transmits two or more message signals over the same line by using a different time interval for each signal.
  2. Time-division multiplexing (TDM) is a communications process that transmits two or more streaming digital signals over a common channel.
  3. TDM is used to transmit two PCM signals in opposite directions over the same transmission line.

 

19. The different access methods which permit many satellite users to operate in parallel through a single transponder without interfering with each other are

1. Frequency Division Multiple Access (FDM

2. Time Division Multiple Access (TDM

3. Code Division Multiple Access (CDM

Which of the above are correct?

  1. 1 and 2 only
  2. 1 and 3 only
  3. 2 and 3 only
  4. 1, 2 and 3

Answer.2. 1 and 3 only 

Explanation

In TDM, each signal occupies the entire bandwidth of the channel. However, each signal is transmitted for only a short period of time.

In TDMA, different users access the transponder in different time slots, not in parallel. In serial transmission is used for transmission over long distances. The communication modes allow for one-way, two-way ordered, or two-way simultaneous transmission.

In serial transmission, the data is sent via a single cable. When a clock pulse is applied to the shift register it transmits the information bit by bit in allocated time slots.

 

20. Example of a reservation system is

  1. Synchronous TDM
  2. Non-synchronous TDM
  3. Synchronous & Non-synchronous TDM
  4. None of the mentioned

Answer.1. Synchronous TDM

Explanation

A Synchronous TDM is a technique in which a time slot is preassigned to every device. In Synchronous TDM, each device is given some time slot irrespective of the fact that the device contains the data or not. If the device does not have any data, then the slot will remain empty.

In the reservation scheme, a node is required to make a reservation prior to transmitting data. Time is split into intervals. In each interval, a reservation frame is initially sent.

Assume that there is n number of nodes. In this case, the reservation frame has precisely n number of mini time slots. Each mini time slot corresponds to a node.

If a node has sufficient data to send, it indicates its intention in its corresponding mini time slot. The nodes that made reservations, then transmit their data frames subsequent to the transmission of the reservation frame.

Synchronous TDM is an example of the reservation system

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